The present invention generally relates to ceramic matrix composite (CMC) components and processes for their production. More particularly, this invention provides a method to reduce corner cracking and delamination of environmental barrier coating (EBC) systems at corners of CMC components.
Higher operating temperatures for gas turbine engines are continuously being sought in order to improve their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of iron, nickel, and cobalt-based superalloys. While superalloys have found wide use for components used throughout gas turbine engines, and especially in the higher temperature sections, alternative lighter-weight component materials have been proposed.
Ceramic matrix composites (CMCs) are a class of materials that include a reinforcing material surrounded by a ceramic matrix phase. Such materials, along with certain monolithic ceramics (i.e., ceramic materials without a reinforcing material), are currently being used for higher temperature applications. These ceramic materials are lightweight compared to superalloys, yet can still provide strength and durability to the component made therefrom. Therefore, such materials are currently being considered for many gas turbine components used in higher temperature sections of gas turbine engines, such as airfoils (e.g., turbines and vanes), combustors, shrouds and other like components, that would benefit from the lighter weight and higher temperature capability these materials can offer.
CMC and monolithic ceramic components can be coated with environmental barrier coatings (EBCs) to protect them from the harsh environment of high temperature engine sections. EBCs can provide a dense, hermetic seal against the corrosive gases in the hot combustion environment, which can rapidly oxidize silicon and silicon carbide in CMCs and monolithic ceramics. Additionally, silicon oxide is not stable in high temperature steam, but is converted to volatile (gaseous) silicon hydroxide species. Thus, EBCs can help prevent dimensional changes in the ceramic component due to such oxidation and volatilization processes. Currently, EBCs are applied using standard, industrial coating processes such as plasma spray (APS) and vapor deposition (i.e. chemical vapor deposition, CVD, and electron beam physical vapor deposition, EBPVD). Thereafter, a heat treatment may be performed to relieve residual stresses created during cooling from elevated application temperatures.
As a nonlimiting example of a CMC component, FIG. 1 schematically represents a bucket 10 of a land-based gas turbine engine of a type used in the power generation industry. As represented in FIG. 1, the bucket 10 comprises an airfoil 12 extending from a shank 14. The bucket 10 is further represented as being equipped with a dovetail 16 formed on its shank 14 by which the bucket 10 can be conventionally anchored to a rotor wheel (not shown) as a result of being received in a complementary slot defined in the circumference of the wheel. The dovetail 16 is conventionally configured to be of the axial entry type, in which the dovetail 16 has a fir tree shape adapted to mate with a complementary-shaped dovetail slot in a rotor wheel. The airfoil 12 of the bucket 10 is directly subjected to the hot gas path within the turbine section of a gas turbine engine. The bucket 10 is also represented as having a platform 18 that forms a portion of the radially inward boundary of the hot gas path and, consequently, experiences very high thermal loads. Other relatively conventional features of the bucket 10 include sealing flanges (angel wings) 19 that project axially away from the forward and aft ends of the shank 14.
The conventional EBC application processes discussed above are prone to induce defects such as through-the-thickness and interfacial cracks, especially at corners due to tensile strain induced by the heat treatment performed after the application process. For example, in reference to FIG. 1, a trailing edge and a leading edge of the airfoil 12 are prone to cracking FIG. 2 depicts a series of photographs of an EBC on a CMC component, such as the bucket 10 represented in FIG. 1, showing the progression of vertical cracking and delamination due to tensile strain. FIG. 3 is a force diagram representing the forces that are present as a result of conventional EBC application processes and influence the occurrence of cracking of an EBC system at its corners. The EBC system will generally extend after one heat treatment cycle in both the circumferential and the radial directions, thus a coating thickness t is represented as experiencing a normal force N and a shear force T on the cross-section of the EBC system, interfacial normal stresses σt, and hoop stress σθ at the corner. The simple free-body diagram represented in FIG. 3 illustrates that under certain conditions, particularly as corners become small and sharp, the interfacial stress σt likely to be tensile stress inducing a positive (tensile) hoop stress σθ. This tensile hoop stress σθ and the tensile interfacial stress σt promote EBC cracking and delamination at sharp corners.
Prior attempts to solve cracking and delamination problems of EBC systems include forming interlocking features between layers of an EBC system, forming strain relief grooves in EBC layers, forming grooved bonding surfaces in EBC layers, forming EBC layers with reinforcing particles, as well as a variety of other methods. As an example, U.S. Pat. No. 4,503,130 to Bosshart et al. discloses a process of applying a graded ceramic coating to a metal substrate. During the coating process, the temperature of the substrate is controlled in predetermined degree for establishing residual stress and strain patterns in the manufactured seal. Substrate heaters are provided for this purpose. Although the prior art above describe their methods as providing for reduced cracking and delamination of EBC systems, improved methods are needed to address the cracking and delamination of EBC systems at corners of CMC components.
In view of the above, there is an ongoing need for methods capable of reducing stresses that can induce cracking and delamination at corners of EBC coating systems.